Bleaching

Bleaching, or whitening, is a series of operations on dyeing, calico-printing, and finishing factories to which the grey goods, i.e., fabrics (especially cotton and linen), are subjected, and which aim to free the fiber from components hindering further processing (plant waxes, resins, fats, dyeing and finishing substances), or substances applied during the sizing of the warp at the weaving factory (starch, fat, glycerin, etc.). Previously, the goods are subjected to singeing to destroy fiber tips giving the fabric a fluffy appearance. Singeing is performed on singeing machines (plate or gas); due to the rapid passage of the fabric over a heated plate or over the flame of burners, only the fiber tips burn. After singeing, the processing of the fabric begins, based on chemical processes. First, desizing occurs—the removal of sizing (starch) by soaking the fabric followed by lying down; multiplying microorganisms convert the starch into a soluble state, after which it is washed out. The next process is boiling out: the removal of fatty and waxy substances. This is achieved by boiling the impregnated caustic alkali goods in special 'buching' kettles at elevated pressure and high t° (140°). The alkali saponifies the fats, converting them into soap, which is easily washed off the goods. The boiled goods are rinsed; such goods are called 'half-white'. To convert it into 'white', it is necessary to destroy the dyeing substance contained in the fibers themselves. The destruction of the dyeing substance is achieved by bleaching, or 'spiriting'—processing the goods with a bleaching lime solution (CaOCl2), which decomposes on the goods with the release of free O (in the presence of carbon dioxide): 2CaOCl2 + CO2 → CaCl2 + CaCO3 + Cl2; 2Cl2 + 2NaOH → 4HCl + O2. As a result, the dyeing substances are destroyed by oxidation. After lying for about five hours after treatment with bleaching lime ('spirit'), the goods, after rinsing, go into 'kislovka'—processing with a weak solution of sulfuric acid, which destroys the finishing substances and removes the remaining lime from the goods: Ca(C10)2 + CaCl2 + 2H2SO4 = 2CaSO4 + 2H2O + 2Cl2. After kislovka, the goods are rinsed, wrung, and then dried on drying drums. Some grades of fabrics undergo additional processing. Those goods which are not sent directly for bleaching, but are intended first to serve as lining for printing, after singeing go to calendering machines ('grey calendering'), where they are calendered onto rollers. Other grades of goods, after bleaching, undergo mercerization—processing with a strong (28-30° Bé) solution of caustic soda, which increases the strength of the goods and enhances their receptivity to dyes. On some grades of goods (zibeline, muslin), pile is carded on carding machines. The greater part of white goods is sheared on shearing machines for the final removal of threads and lint. The finished white goods go either to final finishing, or to dyeing, or to printing. In the sanitary-hygienic aspect, the following moments are important: 1. Singeing. The working conditions here are very arduous: in addition to the effect of elevated t° (25-26° and above) and radiant heat, there is the release of a large amount of dust consisting of particles of goods—grey and singed; since the burning fibers burn incompletely, the air of the premises is saturated with smoke and flying products of incomplete combustion in the form of lint. 2. Loading of buching kettles. Horizontal kettles (Matter-Platt) are loaded by rolling in carts with goods; on our factories vertical kettles are used much more often, into which for laying the goods through narrow holes in the lid, teenagers ('gonyalchiki') climb inside. Inside the kettle they work in a very stuffy atmosphere at t° of 30-32° and relative humidity reaching 100%. The kettle men at the kettles are subjected to the effect of high t° (32°-36° and above) and, to a lesser extent, radiant energy. 3. Preparation of bleaching lime solution—one of the most harmful works. This solution is prepared by hand on most factories. When unloading lime from barrels, the worker has to inhale very caustic bleaching lime dust. When dissolving the latter in water, a large amount of chlorine is released; the worker stands directly next to the vat, stirring with a paddle (these workers, according to the Labor Code, enjoy a 6-hour workday). 4. Workers in the bleaching department (machinists of washing alkaline, spirit, and acid machines, gonyalchiki) are subjected to the effect of dampness (soaking of clothes and shoes) and elevated humidity, which never drops below 80% relative humidity, and often reaches 90% and above. Due to the frequent proximity to buching kettles, t° in the bleaching departments is also significantly higher than the hygienically permissible norm. Furthermore, there is always some amount of chlorine in the air of the bleaching departments, released during the processes of spiriting and kislovka. Usually its amount in the air does not reach a dangerous concentration; but it can accumulate in larger quantities in layers of air close to the floor and under certain unfavorable circumstances can cause poisoning of workers (such cases have been observed repeatedly in our factories). 5. Workers at drying drums are subjected to the effect of high t° (up to 36° and above, in summer up to 40°), and with poor insulation of the drums also to the effect of radiant heat. 6. Grey calendering, shearing, and carding (especially the first two) are dusty processes. During calendering of singed grey goods, a huge amount of dust containing singed fibers is released; during shearing, a very large amount of fine dust is also released. Hygienic measures: 1. For singeing machines—complete enclosure in a chamber covered with a heat-insulating layer with poor thermal conductivity and connected to powerful exhaust ventilation. 2. For buching kettles—gradual transition to horizontal Matter-Platt kettles or to vertical Tiss kettles, where the worker performing the loading stands on a platform outside the kettle; furthermore, good insulation of the kettles and supply-exhaust ventilation. 3. For the process of preparing bleaching lime—mechanization of it or, even better (which is already done on other factories)—transition to bleaching with sodium hypochlorite salt, prepared by electrolytic splitting of table salt (NaCl → Na' + Cl'; Na with water gives NaOH; 2NaOH + Cl2 → NaOCl + NaCl + H2O). In this method, the main harm of the work of solution preparers is completely eliminated. 4. In bleaching departments—installation of smooth floors and drains, proper arrangement and placement of machines and tanks for lying down, installation of powerful supply-exhaust ventilation (air supply from above and suction from below). 5. Drying drums must be enclosed in a chamber covered with a heat-insulating layer with poor thermal conductivity and connected to exhaust. 6. Dusty machines—calendering, shearing, and carding—must be equipped with appropriate local exhaust ventilation. Bleaching of other goods, besides cotton fabrics. - Linen. It is bleached more often in the form of yarn, less often—fabric. Linen is processed with weaker solutions of bleaching lime. The process here is little mechanized; it is very long and consists of multiple boilings, spirtings, kislovkas, and rinsing (up to 20 operations), combined with meadow bleaching. Workers in this process are subjected to the effect of sharp dampness, elevated t°, humidity, and chlorine. The effect of these harmful factors is aggravated by the very poor, from a sanitary point of view, arrangement of most linen bleaching factories. - Wool. Woolen cloth is bleached more often by processing with sodium peroxide or sulfur dioxide (SO2). With the first method, there is the effect of dampness and elevated humidity. With the second, workers perform the unloading after bleaching in respirators (after previously ventilating the chambers). - Silk. Silk fabrics are also bleached with the aid of sulfur dioxide. Thus, in bleaching wool and silk, a new harmfulness appears—the danger of poisoning with sulfur dioxide (see). Lit.: Fedorov F. A., Guide to Bleaching, Dyeing, and Printing of Fabrics, M., 1923; Voznesensky N., Bleaching, M., 1924; Sanitary Survey of the Krasnopresnenskaya Trokhgornaya Manufactory ('Health of Labor and Revolution of the Way of Life', vyp. 4 and 5, M., 1924); Pinsker S. M., Rovenbaum N. D., Dorovatshii V. S., Sanitary Characteristics of Detailed Professions of Dyeing and Calico Printing Production, Moscow, 1924.